This invention concerns a system to obtain summary data of the line spectrum supplied by a spectrum analyser from a Doppler radar. This system uses sixteen- or thirty-two-bit words (more generally 2.sup.m), called elementary raw plots, supplied by a contrast detector connectd to the output of a spectrum analyser and supplies summary data from this received information. For this reason, it will hereinafter be called "plot summarizer". With this type of plot summarizer, it is possible to reduce the amount of calculation needed to determine the speed of the target. This is especially useful in the case of a diversity radar.
Target speed is determined on the basis of the frequency shift caused by movement of that target. In the case of a transmission frequency diversity radar, a different Doppler frequency or shift corresponds to each transmitted signal frequency. If Fe.sub.i is the transmission frequency of index i (where i=1 to N) and if a different repetition, or sampling frequency Fs.sub.i, is associated with each transmmission frequency, it will therefore be necessary to obtain a measurement of the Doppler shifts associated with each pair (Fe.sub.i, Fs.sub.i). However, some measurements of this shift made with a spectrum analyser using different sampling frequencies Fs.sub.i, will themselves differ because of spectrum foldovers. The true frequency shift value associated with that pair can be identified by comparing the N values each associated with one pair (Fe.sub.i, Fs.sub.i), and the speed can thus be determined without ambiguity.
In fact, a single-speed target is detected for several contiguous lines on the spectrum analyser. The measurement will thus not consist of a single frequency value g.sub.i (accurate to K.sub.i.Fs.sub.i), but will consist of a set of n contiguous values {g.sub.ik, k=1 to n}. It is shown (sixteen-point Fourier transform property after Tchebycheff's weighting with sixty-decibel rejection) that a single-speed target is detected for a maximum of five cyclically contiguous lines on the spectrum analyser, that is to say supplies at most five lines at high enough levels for a maximum of five g.sub.ik Doppler shift values (n=5) determined to an accuracy of K.sub.i.Fs.sub.i. In more general terms, for 2.sup.m bits, n=2.sup.m /4+1 will be obtained. So as to limit the amount of calculation required to determine the target's speed, it is desired to reduce these (one to five) contiguous lines in one way or another to a single value g.sub.i, summarizing them. This is the purpose of the plot summarizer which must also differentiate between single-speed targets and other targets, so as to avoid giving summary values having no direct physical significance.